A typical electrophotographic image forming apparatus includes a fixing device for fusing and then fixing a toner powder image supported by a recording medium onto the same substrate. One well known such fixing device is a roller fixing device which includes a heat roller having a heat generator therein and a pressure roller arranged parallel to and in circumferencial contact with the heat roller to form a nipping region therewith. With this roller fixing device, the sheet substrate supporting the unfixed toner powder image is transported into the nipping region where the toner powder image is fused and then fixed onto the recording medium.
However, this roller fixing device has a drawback that the nipping region is so small that the rollers must be rotated in a rather lower speed for fixing the entire toner powder image onto the sheet. This further limits a speed of image forming. One approach for overcoming this drawback is to elevate a pressure applied on the pressure roller towards the heat roller, thereby increasing a fixing ability of the device. This technique is effective for fixing the toner powder image on the substrate sheet, but has serious disadvantage that the toner powder image is flattened by the elevated pressure, which leads a deterioration of the resultant image.
In place of the roller fixing device, a belt fixing device using an endless fixing belt is disclosed in Laid-Open JPA 60-151677. This belt fixing device is shown in FIG. 8. The belt fixing device 100 comprises a fixing roller 101 made of metal. The fixing roller 101 includes a heat generator, or lamp 102, therein. The fixing roller 101 is covered at its outer periphery with an offset-preventing material such as fluoroethylene or silicone. The fixing device 100 also comprises a drive roller 103 drivingly connected with a motor (not shown), a trailing roller 104, and an assist roller 105. An endless belt 106 is entrained around these three rollers 103, 104, and 105. Also, the fixing roller 101 is arranged in circumferential contact with a portion of the belt 106 between the drive roller 103 and the assist roller 105 to form an extended nipping region 107.
With this belt fixing device 100, a recording medium 109 supporting an unfixed toner powder image 110 is transported in a direction indicated by an arrow 111 into the nipping region 107 and then the toner powder image 110 is pressure-fixed by a contact with the fixing roller 101 heated by the lamp 102.
This pressure roller fixing device 100, however, needs a separator for separating the sheet 109 which has moved past the nipping region 107 from the fixing roller 101. Also, to accelerate the fixing and then printing, a portion of the belt 106 that has touched with the recording medium 109 in the nipping region 107 and has been deprived of heat must be re-heated ps soon as possible up to an elevated temperature required for fixing. This requirement prohibits the use of fixing roller made of elastic material such as silicone rubber having low thermal conductivity. Therefore, a metal roller is typically employed for the fixing roller 101 though, this results in the flattening of the fixed toner image to eventually deteriorate the image. Particularly, a full-color toner image which is formed by superimposing a plurality of toner images of different colors is easy to be flattened, which deteriorates the image too much.
Another belt fixing device shown in FIG. 9 is disclosed in Laid-Open JPA 4-324476. This belt fixing device 120 comprises a pair of parallel heat rollers 121 and 122 including respective heaters 123 and 124. An endless belt 125 is entrained around the rollers 121 and 122. A pressure roller 126 is so forced against the rollers 121 and 122 as to be in circumferential contact with a lower span of the belt 125 to form an extended nipping region 127 therewith. Two thermal sensors 128 and 129 are arranged inside the belt 125 to contact with respective peripheral surfaces of the rollers 121 and 122 for detecting temperatures thereof. Outputs of the sensors 128 and 129 are used for switching the heaters 123 and 124, respectively.
Further, the belt 125 is made from a thin walled belt of metal such as nickel having a high thermal conductivity and the outer surface thereof is covered by the offset-preventing material, and the heat rollers 121 and 122 appears to be made of metal having high thermal conductivity. The pressure roller 126 comprises a mandrel (not shown) and a relatively thick elastic layer 130 mounted therearound.
With this belt fixing device 120, a recording medium 131 bearing an unfixed toner image 132 is transported through the nipping region 127. Thereby, the toner image 132 is heated by contact with the heated belt 125 and then fixed on the recording medium 131.
This belt fixing device is also effective for heating the recording medium 131 and the toner image 132, but the metal rollers 63 and 64 are pressed against the toner image on the recording medium 131 through the belt 65. This causes the flattening of toner image and deteriorates the image.